This invention relates to position sensors or encoders such as an optical shaft angle encoder which produces electrical signals indicative of the angular position of the shaft. In particular, it relates to an encoder that provides a unique signal so that the position can be determined absolutely, rather than by reference to distance traveled from an index point or the like.
Incremental optical motion encoders are used to resolve the position and movement of an object along a particular route. Such encoders generally include a light source for emitting a light beam, light modulation means for modulating the light beam in response to movement of the object along the route, and a detector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the detectors. The electrical signals from the detectors can be used to indicate the change in location of the object along the route. Multiple detectors are used to indicate both change in location and direction of movement.
Generally speaking, for an incremental motion encoder to produce an indication of the absolute position or location of the object its route, an index pulse is generated at least once along the route. Thereafter, incremental signals can be used to count incremental movement from the index pulse. When the position of the object is known at the index pulse, the absolute position of the object at any place along the route can be determined. To provide an indication of absolute position, change in location and direction of movement, an incremental encoder generally requires three channels of information. Two channels are derived from two or more out-of-phase encoder signals that are produced throughout the route of the object, and the third is an index signal that is produced at least once along the route and at a known position.
In an exemplary embodiment, such a position encoder or movement detector may be used for measuring the angular position of a shaft. Depending on the use of such a shaft angle encoder, a high degree of resolution and accuracy may be needed. It is not unusual to specify a resolution of 2,000 increments per revolution of the shaft. Accuracy of the correlation between the signal from the encoder and the actual mechanical position of the shaft or other object is also important. Mechanical alignment discrepancies in assembling apparatus can adversely affect accuracy. This may be particularly true where the information, i.e., the position track and the index track, are physically separated from each other. In a shaft angle encoder, the center of each of these tracks must be concentric to a high degree to obtain the desired accuracy.
It is desirable to provide an absolute encoder having a single track for position information. Absolute encoders are known, but are quite expensive.
In a conventional shaft angle encoder where position is obtained by reference to an index location, it is necessary upon "wake-up" of the instrument to search for the index so that absolute position can be determined. It is desirable to have a shaft angle encoder where absolute position can be determined immediately when the encoder is turned on. Thus, it is not necessary to rotate the shaft to find the index so that incremental counts can be made from the index.